Semiconductor devices often have one or more arrays of patterned interconnect levels that serve to electrically couple the individual circuit elements forming an integrated circuit (IC). The interconnect levels are typically separated by an insulating or dielectric coating. Previously, a silicon oxide coating formed using chemical vapor deposition (CVD) or plasma enhanced techniques (PECVD) was the most commonly used material for such dielectric coatings. However, as the size of circuit elements and the spaces between such elements decreases, the relatively high dielectric constant of such silicon oxide coatings (i.e. about 4) is inadequate to provide adequate electrical insulation.
In order to provide a lower dielectric constant than that of silicon oxide, dielectric coatings formed from siloxane-based resins have found use. An example of such coatings are those formed from hydrogen silsesquioxane resins as described for example in Collins et al., U.S. Pat. No. 3,615,272 and Haluska et al. U.S. Pat. No. 4,756,977. While such coatings provide lower dielectric constants than CVD or PECVD silicon oxide coatings and also provide other benefits such as enhanced gap filling and surface planarization, typically the dielectric constants of such coatings are limited to approximately 3 or greater.
It is well known that the dielectric constant of insulating coatings is an important factor where IC's with low power consumption, cross talk, and signal delay are required. As IC dimensions continue to shrink, this factor increases in importance. As a result, siloxane based resin materials and methods for making such materials that can provide electrically insulating coatings with dielectric constants below 3 are desirable. In addition it is desirable to have siloxane-based resins and methods for making such resins that provide coatings which have a high resistance to cracking. Also, it is desirable for such siloxane-based resins to provide coatings by standard processing techniques such as spin coating. It is known that the dielectric constant of solid coatings decrease with a decrease in density of the coating material. A porous coating typically has a lower density than a corresponding solid coating.
Haluska, U.S. Pat. No. 5,446,088 describes a method of co-hydrolyzing silanes of the formulas HSi(OR)3 and Si(OR)4 to form co-hydrolysates useful in the formation of coatings. The R group is an organic group containing 1-20 carbon atoms, which when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent. Especially preferred hydrolyzable groups are methoxy and ethoxy. The hydrolysis with water is carried out in an acidified oxygen containing polar solvent. The co-hydrolyzates in a solvent are applied to a substrate, the solvent evaporated and the coating heated to 50 to 1000° C. to convert the coating to silica. Haluska does not disclose silanes having branched alkoxy groups.
Chung et al., U.S. Pat. No. 6,231,989 describe a method for forming a porous coating from hydrogen silsesquioxane resins. A porous network is formed by depositing a coating on a substrate with a solution comprising a hydrogen silsesquioxane resin and a solvent in a manner in which at least 5 volume % of the solvent remains in the coating after deposition. The coating is then exposed to an environment comprising a basic catalyst and water; the solvent is evaporated from the coating to form a porous network with a dielectric constant in the range of 1.5 to 2.4.
Smith et al., WO 98/49721, describe a process for forming a nanoporous dielectric coating on a substrate. The process comprises the steps of blending an alkoxysilane with a solvent composition and optional water; depositing the mixture onto a substrate while evaporating at least a portion:of the solvent; placing the substrate in a sealed chamber and evacuating the chamber to a pressure below atmospheric pressure; exposing the substrate to water vapor at a pressure below atmospheric pressure and then exposing the substrate to base vapor.
Mikoshiba et al., U.S. Pat. No. 6,022,814, describe a process for forming silicon oxide films on a substrate from hydrogen or methyl siloxane-based resins having organic substituents that are removed at a temperature ranging from 250° C. to the glass transition point of the resin. Silicon oxide film properties reported include a density of 0.8 to 1.4 g/cm3, an average pore diameter of 1 to 3 nm, a surface area of 600 to 1,500 m2/g and a dielectric constant in the range of 2.0 to 3.0. The useful organic substituents that can be oxidized at a temperature of 250° C. or higher that were disclosed include substituted and unsubstituted alkyl or alkoxy groups exemplified by 3,3,3-triflouropropyl, β-phenethyl group, t-butyl group, 2-cyanoethyl group, benzyl group, and vinyl group.
Mikoskiba et al., J. Mat. Chem., 1999, 9, 591-598, report a method to fabricate angstrom size pores in methylsilsesquioxane coatings in order to decrease the density and the dielectric constant of the coatings. Copolymers bearing methyl (trisiloxysilyl) units and alkyl (trisiloxysilyl) units were spin-coated on to a substrate and heated at 250° C. to provide rigid siloxane matrices. The coatings were then heated at 450° C. to 500° C. to remove thermally labile groups and holes were left corresponding to the size of the substituents, having a dielectric constant of about 2.3. Trifluoropropyl, cyanoethyl, phenylethyl, and propyl groups were investigated as the thermally labile substituents.
Ito et al., Japanese Laid-Open Patent (HEI) 5-333553, describe preparation of a siloxane resin containing alkoxy and silanol functionality by the hydrolysis of diacetoxydi(tertiarybutoxy)silane in the presence of a proton acceptor. The resin is radiation cured in the presence of a photo acid with subsequent thermal processing to form an SiO2 like coating and can be used as a photo resist material for IC fabrication.
It has now been found that incorporation of organic groups having 6 to 30 carbon atoms and branched alkoxy groups having 3 to 30 carbon atoms into siloxane resins provides several advantages such as improved storage stability, increased modulus and increased porosity of the cured resins, while retaining a dielectric constant in the range of 1.5 to 3.0. It is therefore an object of this invention to show a siloxane resin composition having improved storage stability. It is also an object of this invention to show a method for making siloxane resins and a method for curing these resins to produce insoluble porous coatings having a dielectric constant of 1.5 to 3.0, a porosity from 1 to 60 volume percent and a modulus from 1.0 to 10 GPa. These coatings have the advantage that they may be formed using conventional thin film processing.